This invention relates to the control of power sources in a parallel AC electrical power system and, more particularly, to circuits and methods for synchronizing the outputs of those power sources.
AC electric power systems are usually connected in parallel to increase total system rating or in certain cases such as airborne power systems, to increase reliability. One well known type of aircraft electric power system is the DC link variable speed constant frequency (VSCF) system. Such systems include a plurality of power pole switching elements which synthesize an AC output by switching current from a pair of DC link conductors in a fixed switching pattern, which may be generated by a microprocessor or other digital circuit. When these systems are to be operated in parallel with each other or with another source such as a ground power cart, the outputs of the power sources must be synchronized. Perfect synchronization requires the matching of both the phase angle and voltage amplitude of the power source outputs.
Prior synchronization circuits controlled the phase angle and voltage amplitude of the power source outputs with independent circuits. Quite often the voltage was not adjusted, but was maintained by the individual set point accuracy of the voltage regulators in the two systems. The phase angle control circuits responded to zero cross-overs of the AC waveform. Waveform distortion could cause significant errors. Because typical inverter and external source impedances are 0.1 per unit or less, small voltage errors can produce significant current transients at the instant of paralleling. Paralleling controls or load division circuits have been developed for inverter power sources as illustrated by U.S. Pat. No. 4,510,399, issued Apr. 9, 1985, and application Ser. Nos. 938,652 (now U.S. Pat. No. 4,707,142, issued Nov. 17, 1987), 938,661 and 938,702 all filed Dec. 5, 1986. These circuits actively control the frequency and voltage of an inverter output to cause the proper sharing of real and reactive load. It is desirable to include a synchronization circuit which is compatible with existing load division and control circuits.